title: Grazer cues induce stealth behavior in marine dinoflagellates.
abstract: Chain formation is common among phytoplankton organisms but the underlying reasons and consequences are poorly understood. Here we show that chain formation is strongly impaired by waterborne cues from copepod grazers in the dinoflagellate Alexandrium tamarense. Chains of Alexandrium cells exposed to copepod cues responded by splitting into single cells or shorter chains. Motion analysis revealed significantly lower swimming velocities for single cells compared with chains, with two- to fivefold higher simulated predator encounter rates for two- and four-cell chains, respectively. In addition, the few remaining two-cell chains in grazed treatments were swimming at approximately half the speed of two-cell chains in treatments without grazers, which reduced encounter rates with grazers to values similar to that of single cells. Chain length plasticity and swimming behavior constitute unique mechanisms to reduce encounters with grazers. We argue that dinoflagellates can regulate the balance between motility and predator avoidance by adjusting chain length. The high predator encounter rate for motile chains may have contributed to the low prevalence of chain formation in motile phytoplankton compared with in nonmotile phytoplankton where chain formation is more common.
authors.affiliation: Section for Ocean Ecology and Climate, National Institute for Aquatic Resources, Technical University of Denmark, Kavalergården 6, DK-2920 Charlottenlund, Denmark. erse@aqua.dtu.dk
authors.forename: Erik
authors.initials: E
authors.lastname: Selander
authors.forename: Hans H
authors.initials: HH
authors.lastname: Jakobsen
authors.forename: Fabien
authors.initials: F
authors.lastname: Lombard
authors.forename: Thomas
authors.initials: T
authors.lastname: Kiørboe
mesh_terms: D004141:Dinoflagellida / Q000502:physiology*; D008386:Marine Biology; D013550:Swimming